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Toric IOLs beneficial for cataract patients with corneal astigmatism
With the lens properly aligned, the total astigmatism of the eye can be effectively reduced or even eliminated, resulting in better visual acuity.
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With refractive cataract surgery, it is important to address corneal astigmatism at the time of the procedure. For small degrees of corneal astigmatism, making the clear corneal phaco incision at the steep meridian is often sufficient. For more significant amounts of cylinder, limbal relaxing incisions can address corneal astigmatism effectively.
 Uday Devgan |
Toric IOLs are another option for cataract patients with significant levels of corneal astigmatism. With the toric IOL properly aligned with the corneal astigmatism, the total astigmatism of the eye can be effectively reduced or even eliminated, resulting in better visual acuity for the patient without the need for glasses or contacts. Keep in mind, however, if your patient has been wearing rigid gas permeable contact lenses for many decades, future use of the RGP contact can still reduce the corneal astigmatism, but it may unmask the astigmatic correction of the toric IOL.
The two primary U.S. Food and Drug Administration-approved toric IOLs are the STAAR Surgical TL-series toric IOLs, made of silicone and correcting 1.5 D to 3.5 D of corneal astigmatism, and the Alcon AcrySof toric IOLs, made of acrylic and correcting 1 D to 2 D of corneal astigmatism. The STAAR toric was the first FDA-approved toric IOL, and the upgraded TL design is longer in length and has demonstrated more stability within the eye as compared with the original design. Other manufacturers will certainly release competing toric IOLs in the near future.
Clinical exam and calculation of IOL power
For toric IOLs, the spherical dioptric power of the lens is determined in the same way as for nontoric IOLs because the IOL formulae use the average corneal power for the calculations. To determine the amount and the axis of corneal astigmatism, using a manual keratometer is often best, with the corneal topographer used as confirmation. A toric IOL that is misaligned will reduce the effectiveness of the cylinder reduction and may even induce distortion and a new axis of astigmatism, which the patient may find uncomfortable.
The clear corneal incision used during phacoemulsification has an effect on the corneal astigmatism, typically a flattening at the meridian of the incision of about 0.3 D to 0.5 D. This must be accounted for when implanting the toric IOL – placing the incision on the same axis of the astigmatism will enhance the effect of the toric IOL, whereas placing it 90° apart will reduce the effect.
Intraoperative techniques
Marking the cornea is an important part of properly aligning the toric IOL, and this should be done before surgery while the patient is sitting up. When the patient lies flat on the operating room bed, there can be cyclotorsion of the eye and a significant misalignment if the cornea has not been marked previously. Marking the superior and inferior 90° meridians (12 and 6 o’clock positions) is the standard method before surgery, with other axes marked intraoperatively with a Mendez gauge.
 With a toric IOL in the capsular bag, the chopper is used to hold the IOL securely in correct alignment while the irrigation and aspiration probe is used to remove viscoelastic from the anterior chamber.  At the end of the surgery, the toric IOL is properly aligned with the 90° marking at the limbus. Note that the capsulorrhexis is well-sized and completely overlaps the optic edge, further adding to stability of the lens. Images: Devgan U |
The toric IOL must stay in the correct orientation after the cataract surgery. This can be achieved by ensuring that the capsulorrhexis is smaller than the optic so that it securely holds the IOL in the capsular bag. Viscoelastic should be removed from behind the toric IOL. Then, while removing viscoelastic from the anterior chamber, a chopper or second instrument can be used to hold the IOL in position (Figure 1). A light tapping motion will help to secure the IOL in position, and then incisions can be sealed.
At the end of the case, confirmation of the toric IOL alignment should be confirmed (Figure 2), and the patient should be instructed to avoid eye rubbing or ocular trauma. If the IOL is noted to be rotated or misaligned during the first few postoperative exams, it should be repositioned in a timely manner. For every 10° of misalignment, the toric IOL loses about 33% of its effectiveness. It has essentially no ability to address the astigmatism if rotated by 30°, and further rotation induces oblique astigmatism at a new axis.
Toric IOLs can be an effective way to address corneal astigmatism at the time of cataract surgery. With current IOLs, we can address up to 3.5 D of corneal cylinder, and by combining toric IOLs with limbal relaxing incisions, we can address even more.
For more information:
- Uday Devgan, MD, FACS, is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills, and Newport Beach, California. Dr. Devgan is Chief of Ophthalmology at Olive View UCLA Medical Center and an Associate Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine. Dr. Devgan can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax: 310-388-3028; e-mail: devgan@gmail.com; Web site: www.DevganEye.com. Dr.Devgan is a consultant to Abbott Medical Optics and Bausch & Lomb, and is a stockholder in Alcon Laboratories and formerly in Advanced Medical Optics.
References:
- Gills J, Van der Karr M, Cherchio M. Combined toric intraocular lens implantation and relaxing incisions to reduce high preexisting astigmatism. J Cataract Refract Surg. 2002;28:1585-1588.
- Hill W. Expected effects of surgically induced astigmatism on AcrySof toric intraocular lens results. J Cataract Refract Surg. 2008;34:364-367.
- Werner L, Olson RJ, Mamalis N. New technology IOL optics. Ophthalmol Clin North Am. 2006;19: 469-483.